Controlling the impedance of an output driver of a semiconductor chip is desirable in order to minimize the impact of the adverse transmission line effects on the propagated signal. If the impedance is not controlled in this environment, the receiving chip may see a spurious transition and misinterpret the transmitted data.
Existing conventional techniques for controlling the impedance generally fall into one of three categories. The first conventional technique involves centering the design on the typical process corner and then tolerating any process variation. Unfortunately, this technique typically only works for low speed interfaces where the signal is allowed to settle out before sampling. Additionally, this technique often cannot be made to work for higher performance interfaces as process variability can be too great to overcome without resorting to some form of calibration.
The second conventional technique involves using one or more off-chip precision components (e.g., a resistor) on which a calibration loop operates. This technique uses calibration to remove process, temperature, and/or supply voltage sensitivity from one or more parameters in the circuit. The calibration is done using either a digital or analog feedback loop that compares a tunable on-chip component with a corresponding off-chip precision component. The on-chip component matches the off-chip component when calibration is completed. Unfortunately, this technique involves the additional expense of the off-chip precision component (e.g., resistor) along with the greater expense associated with installing that off-chip component onto a circuit board.
The third conventional technique involves using fuses to encode process information. This technique is done by measuring one or more process parameters and then blowing fuses to encode this information. The third technique is a subset of the second technique in the sense that it too is calibrating, but the calibration is usually only run once. Unfortunately, the extra processing step of blowing the fuses during manufacturing can incur additional expense along with reliability issues. For example, it can be difficult to determine if a fuse was successfully blown.